Need Help with RFT Alpha-Uni

[LarryInMichigan]

I am trying to restore an East German RFT Alpha-Uni phone that I recently bought, and the volume from the receiver is much too low.  The receiver capsule works well with other phones, so I know that the problem is not with it.  There are a number of jumper wires on the PC board, so I am wondering if the volume issue can be corrected by moving a jumper or two.  I am using an Ooma Telo device, so the loop current is quite low.  I am attaching some low quality scans of the wiring diagram and schematic for this model that I found.  Can someone here point out what I can change?

Thank You

Larry

[tubaman]

I'd start with X12 as it mentions line length on the diagram.
I think it says 'On XN for 0-2km line length' and 'On XH for 2-5km line length'.

[LarryInMichigan]

I'd start with X12 as it mentions line length on the diagram.
I think it says 'On XN for 0-2km line length' and 'On XH for 2-5km line length'.

I tried moving that jumper, but it didn't make a difference.

Thank You

Larry

[countryman]

Does any of the electrolytic capacitors look suspicious, like showing signs of leakage or swelling?
Is the transmitter the usual carbon type? They started experimenting with amplified piezo capsules by that time... if carbon, might a way too high resistance result in low loop current and malfunction of the transistor circuit? At least the black plastic GDR carbon transmitters now are all as deaf as the communist party leaders were for the people's voice.
It's hard to find out anything about this solid state speaking circuit, made with conventional components (not integrated circuits). The German forum does not yield much more than what you found already...

[LarryInMichigan]

The electrolytic capacitors all seem to look alright.  The biggest one, 1 µF, is printed in Cyrillic characters.  I could conceivably replace the capacitors.  The transmitter seems to be completely dead. 

The communist "leaders" never had any interest in listening to the voice of the people beyond what was necessary to keep themselves in power.

Thank You

Larry

[countryman]

Have you tried any other carbon transmitter? Possibly connected via jumper leads...

[LarryInMichigan]

Yes, I replaced the bad transmitter with a working carbon WE T1.

Larry

[TelePlay]

Yes, I replaced the bad transmitter with a working carbon WE T1.

With what results?

[countryman]

I'm afraid with no improvement if not mentioned otherwise?
Looks like you have to work it the hard way and test the components one by one... the transistors should be silicone types already, and easy to replace with common western ones.

[LarryInMichigan]

The volume from the receiver is very low when either receiver is used.

Larry

[countryman]

Don't hesitate to post on the German forum, you might find a knowledgeable person there more likely than anywhere else.
You can write in English and/or use Google translate. The results are pretty good normally for EN/DE translations.
https://www.wasser.de/telefon-alt/forum/index.pl